This invention is in the area of organic chemistry, and in particular provides a stereoselective synthesis of 1,3-dioxolane nucleosides.
In 1981, acquired immune deficiency syndrome (AIDS) was identified as a disease that severely compromises the human immune system, and that almost without exception leads to death. In 1983, the etiological cause of AIDS was determined to be the human immunodeficiency virus (HIV). In December, 1990, the World Health Organization estimated that between 8 and 10 million people worldwide were infected with HIV, and of that number, between 1,000,000 and 1,400,000 were in the United States.
In 1985, it was reported that the synthetic nucleoside 3'-azido-3'-deoxythymidine (AZT) inhibits the replication of human immunodeficiency virus type 1. Since then, a number of other synthetic nucleosides, including 2', 3'-dideoxyinosine (DDI), 2',3'-dideoxycytidine (DDC), 3'-fluoro-3'-deoxythymidine (FLT), 2', 3'-dideoxy,2', 3'-didehydrothymidine (D4T), and 3'-azido-2', 3'-dideoxyuridine (AZDU), have been proven to be effective against HIV. A number of other 2', 3'-dideoxynucleosides have been demonstrated to inhibit the growth of a variety of other viruses in vitro. It appears that, after cellular phosphorylation to the 5'-triphosphate by cellular kinases, these synthetic nucleosides are incorporated into a growing strand of viral DNA, causing chain termination due to the absence of the 3'-hydroxyl group.
Both DDC and D4T are potent inhibitors of HIV replication with activities comparable (D4T) or superior (DDC) to AZT. However, both DDC and D4T are not efficiently converted to the corresponding 5'-triphosphates in vivo and are resistent to deaminases and phosphorylases. Both compounds are also toxic.
The success of various 2', 3'-dideoxynucleosides in inhibiting the replication of HIV in vivo or in vitro has led a number of researchers to design and test nucleosides that substitute a heteroatom for the carbon atom at the 3'-position of the nucleoside. Norbeck, et al., disclose that (.+-.)-1-[(2.beta., 4.beta.)-2-(hydroxymethyl)-4-dioxolanyl]thymine (referred to below as (.+-.)-dioxolane-T or DOT, see FIG. 2), a 1,3 dioxolane nucleoside, exhibits a modest activity against HIV (EC.sub.50 of 20 .mu.m in ATH8 cells), and is not toxic to uninfected control cells at a concentration of 200 .mu.m. Tetrahedron Letters 30 (46), 6246, (1989). In light of the fact that this compound exhibits efficacy against HIV and has very low toxicity, it is desirable to develop suitable synthetic protocols for preparing a wide variety of analogs and isomers of this compound.
To market a nucleoside for pharmaceutical purposes, it must not only be efficacious with low toxicity, it must also be cost effective to manufacture. An extensive amount of research and development has been directed toward new, low cost processes for large scale nucleoside production. 2', 3'-Dideoxynucleosides are currently prepared by either of two routes; derivatization of an intact nucleoside or condensation of a derivatized sugar moiety with a heterocyclic base. Although there are numerous disadvantages associated with obtaining new nucleoside analogues by modifying intact nucleosides, a major advantage of this approach is that the appropriate absolute stereochemistry has already been set by nature. Obviously, this approach cannot be used in the production of nucleosides that contain either nonnaturally occurring bases or nonnaturally occurring carbohydrate moieties (and which therefore are not prepared from intact nucleosides), such as 1,3-dioxolane nucleosides.
When condensing a carbohydrate-like moiety such as a 1,3-dioxolane with a heterocyclic base to form a synthetic nucleoside, a nucleoside is produced that has two chiral centers (at the C1' and C4' positions, see FIG. 1), and thus exists as a diastereomeric pair. Each diastereomer exists as a set of enantiomers. Therefore, the product is a mixture of four enantiomers. It is often found that nucleosides with nonnaturally-occurring stereochemistry in either the C1' or the C4'-positions are less active than the same nucleoside with the stereochemistry as set by nature.
It is well known in the art that the stereoselective introduction of bases to the anomeric centers of carbohydrates can be controlled by capitalizing on the neighboring group participation of a 2-substituent on the carbohydrate ring [Chem. Ber. 114:1234 (1981)]. However, dioxolanes do not possess an exocyclic 2-substituent and, therefore, cannot utilize this procedure unless additional steps to introduce a functional group that is both directing and disposable are incorporated into the synthesis. These added steps would lower the overall efficiency of the synthesis.
It is also well known in the art that "considerable amounts of the undesired .beta.-nucleosides are always formed during the synthesis of 2'-deoxyribosides" [Chem. Ber. 114:1234, 1244 (1981)]. Furthermore, this reference teaches that the use of simple Friedel-Crafts catalysts like SnCl.sub.4 in nucleoside syntheses produces undesirable emulsions upon the workup of the reaction mixture, generates complex mixtures of the .alpha. and .beta.-isomers, and leads to stable .delta.-complexes between the SnCl.sub.4 and the more basic silylated heterocycles such as silylated cytosine. These complexes lead to longer reaction times, lower yields, and production of the undesired unnatural N-3-nucleosides.
Therefore, it is an object of the present invention to provide a method for the synthesis of a variety of 1,3-dioxolane nucleosides that includes condensing a 1,3-dioxolane moiety with a purine or pyrimidine base through a process that provides high .beta.-stereoselectivity at the C1' position.
It is another object of the present invention to provide a method for the resolution of racemic mixtures of 1,3-dioxolane nucleosides at the C4'-position.